People are paying for prescriptions that should be free, according to new findings from an ongoing study.

Once a family or person has paid for 20 prescriptions, they are supposed to be exempt from the $3 prescription fee.

However, researchers at Victoria and Otago universities found many people were paying for medication that should have been free.

"In fact, the average amount people paid for medicines hardly dropped at all after they reached 20 items,'' Victoria University associate professor Jackie Cumming said.

Pharmac data shows that 180,000 people pay for prescriptions after they should be exempt, costing people an extra $2.5 million a year.

Standard charges for prescription medicines will go up from $3 to $5 in January 2013.

In theory, a person or family will not have to pay more than $100 a year for prescriptions.

But University of Otago professor Pauline Norris said people will potentially pay more.

"Given that the exemption after 20 items does not seem to work in practice, people with multiple health problems, who use a lot of prescription medicines, will potentially have to pay much more than that".

New Zealand and overseas studies had showed that prescription charges lead to less use of medicines and poorer health outcomes, Norris said.

People need to have a prescription subsidy card to be exempt from charges. They also need to have one main pharmacy and take receipts from other pharmacies they visit to their main one.

''Vulnerable people, such as those with multiple health problems, the elderly and people with low health literacy, may particularly struggle with these bureaucratic procedures.

''These are the very people who we need to ensure get the health care they need," Norris said.

The study into equity in prescription medicine also found the majority of people who had more than 20 items dispensed to them by a pharmacist in a year were from the most socio-economic deprived areas.

Researchers used anonymous data from community pharmacy computers to identify individuals who had more than 20 prescription items in a year.

Results probably underestimated the extent of the problem because families should not pay the $3 fee once they have had 20 prescription items, Otago's Dr Simon Horsburgh said.

"Neither our study or Pharmac can identify families from the data, so these estimates are based on individuals. Because many of these people will have family members who also get prescriptions they should be receiving free prescriptions after fewer than 20 items."